Diffraction elements are general-purpose optical elements widely used for the purpose of spectrally splitting light or dividing luminous flux in the field of spectroscopic optics. The diffraction elements are classified into some types according the shape thereof. In general, they are classified into an amplitude type diffraction element in which light-transmitting parts and non-light-transmitting parts are periodically arranged; and a phase type diffraction element in which grooves are periodically formed on a high-transmittance material. Separately from this classification, they may be classified into a transmission type diffraction element and a reflection type diffraction element according to the direction in which the diffracted light is generated.
With the conventional diffraction elements as described above, the diffracted light obtained when natural light (unpolarized light) is made incident thereon is restricted to unpolarized light. With a polarizing optical element frequently used in the fields of spectroscopic optics, only unpolarized light can be obtained as the diffracted light. For this reason, there is generally employed a method in which the natural light emitted from a light source is spectrally split by a diffraction element, and further, in order to utilize only a specific polarization component contained therein, the diffracted light is allowed to pass through a polarizer to be used. With this method, there is a problem that the quantity of light is reduced by half because about 50 percent of the resulting diffracted light is absorbed by the polarizer. Therefore, it is also required to prepare a detector with a high sensitivity and a light source capable of producing a large quantity of light. Under such circumstances, there has arisen a demand for the development of such a diffraction element that the diffracted light itself is specific polarized light such as circularly polarized light or linearly polarized light.